Comparative movement of labelled nitrogen and zinc in 1 ... - INTA

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840 greenhouse-grown seedlings showed that less than 1% of the Zn applied as ZnSO 4 or as Zn metalosate was absorbed by the leaf tissue. In apples, studies using both unlabelled Zn and the stable isotope, 68 Zn, demonstrated that neither dormant nor Autumn applications contributed significantly to the Zn content of newly-developing tissues in the Spring (Sánchez and Righetti, 2002). However, Spring applications of Zn affected only those leaves contacted, and very little Zn moved from the sprayed leaves to new organs. Thus, it appears that foliar-applied Zn is rather immobile. The fact that symptoms of Zn deficiency are clearly visible in new growth is consistent with the limited mobility of Zn applied to mature foliage. Thus, the efficacy of Zn application will depend on the percentage of tree leaf area that receives the Zn spray, and on the physiological age of the foliage exposed. Up to now, no attempts have been made to quantify the amount of Zn delivered to storage tissues after foliar application, or to assess the portion of the storage pool of Zn that is remobilised into new growth the following Spring. In contrast to Zn, the relatively high N-use efficiency of foliar urea sprays has been welldocumented in many fruit species (Weinbaum, 1988; Rosecrance et al., 1998;Tagliavini et al., 1998;Toselli et al., 2004). However, an assessment of N-recovery in the following season, after a late season application of urea, has not been quantified under orchard conditions in any fruit tree species. Our objectives were (i) to quantify the movement of N and Zn out of leaves following foliar application; (ii) to quantify the distribution of N and Zn in the permanent structures of the dormant tree; and (iii) to quantify the redistribution of foliar-applied N and Zn from the tree structure to new growth in the following Spring. The relative contribution of the movement of foliar-applied Zn to the storage pool should permit a more quantitative perspective on the value of foliar applications of Zn. MATERIALS AND METHODS: In Spring 2003, dormant, nursery-grown peach trees of cv. ‘O’Henry/Nemaguard’ were planted in a sandy loam soil of the Hanford series (typic xerothents) at the University of California Kearney Agricultural Center, Parlier, California (36°48’ N, 119°30’ W). The trees were trained to a Kearney Perpendicular “V” system (DeJong et al., 1994) with two main scaffolds. All shoots not arising from the scaffolds were removed in late Summer. On 11 September 2003, five replicates of three uniform trees that had grown to a height of approx. 1.5 m were selected for treatment. Ten ml of a ZnSO4 solution containing 68 Zn (99.4 Atom % 68 Zn) at 1 mg ml –1 plus 15 N-labelled urea (10.8 Atom % 15 N) at 2% (w/v) was applied with a small paint brush to the adaxial leaf surface of the entire canopy on each of the 15 trees. Before application, the trunks and soil were covered with plastic to prevent contamination by labelled Zn and N from the treated leaves, although care was taken to prevent the labelled solution from dripping or running-off the leaves. As the trees were still growing, a second application of 3 ml per tree, mostly to the new growth, was made on 29 October 2003, following Foliar application of labelled N and Zn in peach the same procedure. A total of 13 ml per tree was thus applied in two applications. Senescent leaves were collected on tarpaulins placed under the trees and subsequently removed from the orchard to avoid any possible contamination. During dormancy (16 January 2004), one tree from each replicate was excavated and separated into roots, trunk and shoots. Backhoe trenches 2.0 m deep were dug 1.5 m from the trunk and the soil around the roots was removed with a pitchfork. We estimated ≥ 90% recovery of root biomass. The two remaining trees in each replicate were excavated on two dates in Spring 2004. All trees were left unpruned between the time of Zn/N application and excavation, to minimise variation between trees and to maximise the recovery of both isotopes. The first Spring excavation was on 22 March, 2 weeks after full bloom, and the second excavation was on 5 April, approx. 4 weeks after full bloom. In both Spring excavations, the trees were divided into roots, trunk, shoots and new growth. Trunks were cut at the graft union, and roots were washed with water at the excavation site. After tissue separation, all fresh materials was taken to the laboratory and washed. Tissues were cut into smaller pieces to facilitate drying, placed in paper bags, dried at 60°C for at least 48 h, and ground to pass a 40-mesh screen. Dry tissue (0.5 g) was digested using a nitric acid-hydrogen peroxide microwave digestion to produce a liquid sample for total Zn and N isotope analysis (Sah and Miller, 1992). Sample analysis Digested plant samples were analysed using an Agilent Technologies 7500c ICP-MS (Agilent Technologies, Palo Alto, CA, USA). The Zn isotopes measured were 64, 66, 67, 68 and 70. Each isotope was monitored for 900 ms with a total of ten replicates per analysis. Yttrium (0.5 µg l –1 ) was added as an internal standard to each blank, each calibration standard, and each plant sample. Calibration reference Zn standards (SPEX CertiPrep, Metuchen, NJ, USA) ranged from 5 ng ml –1 to 5 µg ml –1 total Zn (Zhang and Brown, 1999). To calculate isotopic concentrations, the ion intensities of each Zn isotope were first standardised with the signal of the internal standard. The abundance of each isotope was then determined by normalising each Zn isotope ratio to the total amount of Zn measured in the sample. Isotopic concentrations were calculated using calibration curves for each Zn isotope and the abundance of that isotope in the sample. The enrichment of 68 Zn was evaluated using the 68 Zn/ 67 Zn ratio, which was calculated using the ion intensities (Zhang and Brown, 1999). Standard solutions were analysed throughout the sample run and used to apply a mass discrimination correction factor to all samples. This correction factor was the ratio of the measured 68 Zn/ 67 Zn ratio to the theoretical ratio, and ranged from 0.9944 – 1.0249. The amount of Zn derived from the labelled fertiliser was calculated from the equation based on the isotope ratio modified from Ziegler et al. (1989), described in detail by Zhang and Brown (1999). Nitrogen isotope composition was determined by mass spectrometry. Atom percentage values of 15 N were converted to N derived from the labelled fertiliser using standard conversions (Hauck and Bremer, 1976). Results for dry mass (DM) and the distribution of
TABLE I Biomass distribution (g DW tree –1 ) among organs of trees sampled during dormancy (16 January), 2 weeks after full bloom (22 March) and 1 month after full bloom (5 April) Organ January 16 March 22 April 5 New Growth – 112.8 ± 22.6 339.4 ± 34.9 Shoots 292.0 ± 39.9 a total and labelled Zn and N are reported as means ± standard deviation for all components. Percentage distributions of total and labelled Zn and N for each tissue and sampling date were analysed by the paired Student’s t test. Percentage values were arcsin transformed before statistical analysis. Recovery of Zn and N was calculated as a percentage of the total application of 68 Zn and 15 N isotopes. RESULTS Almost 55% of the total DM of dormant, 1-year-old trees corresponded to roots, 18% to the trunk, and 27% to shoots (Table I). Two weeks after full bloom (22 March 2004), the second batch of trees was excavated, and the total tree weights had not increased over the 65 d since the previous, dormant season excavation (Table I). The main difference in DM distribution between the first and second excavated trees was in the roots. Roots, as a percentage of tree DM, decreased from 55% during dormancy to about 45% and 34%, 2 weeks and 4 weeks after full bloom, respectively. As new growth increased, tree DM increased by approx. 20% between dormancy and 1 month after full bloom (Table I). Total N content increased from 15 g to 26 g per tree between dormancy and 1 month following full bloom. Total Zn content increased from 11 mg to 20 mg per tree during the same period (Table II). Most of the uptake of N and Zn from the soil between dormancy and 4 weeks after full bloom occurred over the 2 week period from E. E. SANCHEZ,S.A.WEINBAUM and R. S. JOHNSON 223.8 ± 7.1 310.0 ± 41.3 Trunk 199.2 ± 54.6 204.0 ± 26.3 210.2 ± 30.0 Roots 603.4 ± 81.0 437.8 ± 23.2 442.4 ± 81.7 Whole Tree 1094.6 ± 145.5 978.4 ± 130.4 1302.0 ± 140.4 a Values are means ± SD. (n = 5 trees). 841 2 – 4 weeks after bloom. The roots of dormant trees contained 70% and 41% of total tree N and Zn contents respectively, at the time when roots comprised 55% of the tree biomass (Table III). In the second excavation, 2 weeks after bud break (22 March), the total N and Zn contents per tree remained relatively unchanged from the levels determined previously in dormant trees. The fact that, in these trees, new growth contained 34% and 38% of the total pool of N and Zn, respectively, 2 weeks after bud break (Table III), indicates the substantial intra-tree redistribution of nutrients that occurs during this period. Recovery of labelled N and Zn applied to trees varied from 45.3 – 49.2% and from 6.8 – 7.9%, respectively (Table IV). These data did not include the 15 N and 68 Zn retained in senescent leaves, as our aim was to quantify the amount of foliar-applied nutrient translocated from leaves to storage in perennial parts of the tree over the Winter. Both elements were translocated throughout the trees after application. Sixty-five percent of the labelled N, and 42% of the labelled Zn were found in roots during dormancy (Table V). Two weeks after full bloom, 24% of the labelled N and 45.5% of the labelled Zn had been translocated to new growth and, by 4 weeks after full bloom (5 April), the storage pool had contributed 38% and 55.9% of N and Zn, respectively, to the new growth. A striking quantitative difference was apparent between the two nutrients in their re-translocation from storage to new growth. Roots remobilised 30% of stored N to the new growth, and 79% of stored Zn (Table IV). As a result, the amount of labelled N in roots decreased from 65% to 43.8% of the tree total, and labelled Zn decreased from 42% to 7.7% of the tree total 1 month after full bloom (Table V). DISCUSSION Previous studies applying 68 Zn to a single leaf or group of leaves in fruit trees have suggested limited mobility (Zhang and Brown, 1999; Sánchez and Righetti, 2002). TABLE II Total tree N (g N tree –1 ) and Zn (mg Zn tree –1 ) distributions among different organs of trees sampled during dormancy (16 January), 2 weeks after full bloom (22 March) and 1 month following full bloom (5 April) January 16 March 22 April 5 Organ N Zn N Zn N Zn New Growth – – 5.70 ± 1.21 4.55 ± 0.86 14.93 ± 1.60 11.32 ± 0.98 Shoots 3.30 ± 0.44 a 4.51 ± 0.78 1.82 ± 0.55 2.95 ± 1.26 2.15 ± 0.24 4.26 ± 0.50 Trunk 1.20 ± 0.35 2.27 ± 0.75 0.89 ± 0.16 1.95 ± 0.79 0.85 ± 0.14 2.13 ± 0.66 Roots 10.79 ± 1.95 4.66 ± 0.36 8.24 ± 0.49 2.69 ± 0.39 7.94 ± 1.29 2.11 ± 0.49 Whole Tree 15.29 ± 2.50 11.44 ± 1.78 16.65 ± 2.05 12.13 ± 2.75 25.87 ± 2.49 19.83 ± 2.26 a Values are means ± SD (n = 5). TABLE III Percentage distribution of total tree N and Zn among different organs of trees sampled during dormancy (16 January), 2 weeks after full bloom (22 March) and 1 month after full bloom (5 April) January 16 March 22 April 5 Organ N Zn Sig. N Zn Sig. N Zn Sig. New Growth – – – 34.0 ± 3.5 37.9 ± 3.1 NS 57.7 ± 2.71 57.3 ± 2.0 NS Shoots 21.8 ± 2.3 a 39.4 ± 2.8 ** b 10.7 ± 2.1 23.6 ± 5.6 ** 8.4 ± 1.1 21.5 ± 1.8 ** Trunk 7.8 ± 1.9 19.5 ± 3.2 ** 5.4 ± 0.6 15.8 ± 4.3 ** 3.3 ± 0.5 10.6 ± 2.6 ** Roots 70.4 ± 2.2 41.1 ± 3.9 ** 49.9 ± 5.3 22.7 ± 4.0 ** 30.6 ± 3.6 10.6 ± 1.5 ** Whole Tree 100.0 100.0 100.0 100.0 100.0 100.0 a Values are means ± SD (n = 5). b NS, **, non-significant, or significant at P ≤ 0.01, respectively, by paired t test.
Page 1: Journal of Horticultural Science &
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